Fix: abort GRFFileScanner on exiting the game as soon as possible

This prevents the window from "freezing" when you close it during
the scanning of NewGRFs, as it first would continue the action.

src/openttd.cpp

@@ -1469,6 +1469,8 @@ void GameLoop()
 		ScanNewGRFFiles(_request_newgrf_scan_callback);
 		_request_newgrf_scan = false;
 		_request_newgrf_scan_callback = nullptr;
+		/* In case someone closed the game during our scan, don't do anything else. */
+		if (_exit_game) return;
 	}
 
 	ProcessAsyncSaveFinish();